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Dark Matter Possibilities Dark Matter Possibilities We now delve into possible explanations for this dark matter problem. Two readings on explanations for Dark Matter: 1. "Miraculous WIMPs", by Manuel Gnida, Symmetry Magazine, July 2015. https://www.symmetrymagazine.org/article/july-2015/miraculous-wimps 2. "Existence and Nature of Dark Matter in the Universe," Virginia Trimble, Ann. Rev. Astron. Astrophys. 25, 452-72 (1987). Section 1 (pp. 425-7) and Sections 6 and 7 (pp. 452-62). The full article and references are on the course web site. Artwork by Sandbox Studio, Chicago with Ana Kova Miraculous 07/15/15 | By Manuel Gnida What are WIMPs, and what makes WIMPs them such popular dark matter candidates? Invisible dark matter accounts for 85 percent of all matter in the universe, aLecting the motion of galaxies, bending the path of light and inuencing the structure of the entire cosmos. Yet we don’t know much for certain about its nature. Most dark matter experiments are searching for a type of particles called WIMPs, or weakly interacting massive particles. “Weakly interacting” means that WIMPs barely ever “talk” to regular matter. They don’t often bump into other matter and also don’t emit light—properties that could explain why researchers haven’t been able to detect them yet. Created in the early universe, they would be heavy (“massive”) and slow-moving enough to gravitationally clump together and form structures observed in today’s universe. Scientists predict that dark matter is made of particles. But that assumption is based on what they know about the nature of regular matter, which makes up only about 4 percent of the universe. WIMPs advanced in popularity in the late 1970s and early 1980s when scientists realiZed that particles that naturally pop out in models of Supersymmetry could potentially explain the seemingly unrelated cosmic mystery of dark matter. Supersymmetry, developed to [ll gaps in our understanding of known particles and forces, postulates that each fundamental particle has a yet-to-be-discovered superpartner. It turns out that the lightest one of the bunch has properties that make it a top contender for dark matter. “The lightest supersymmetric WIMP is stable and is not allowed to decay into other particles,” says theoretical physicist Tim Tait of the University of California, Irvine. “Once created in the big bang, many of these WIMPs would therefore still be around today and could have gone unnoticed because they rarely produce a detectable signal.” When researchers use the properties of the lightest supersymmetric particle to calculate how many of them would still be around today, they end up with a number that matches closely the amount of dark matter experimentally observed—a link referred to as the “WIMP miracle.” Many researchers believe it could be more than coincidence. “But WIMPs are also popular because we know how to look for them,” says dark matter hunter Thomas Shutt of Stanford University and SLAC National Accelerator Laboratory. “After years of developments, we [nally know how to build detectors that have a chance of catching a glimpse of them.” (https://www.symmetrymagaZine.org/sites/default/les/images/standard/WIMPsLUX.jpg) Shutt is co-founder of the LUX experiment and one of the key [gures in the development of the next-generation LUX-ZEPLIN experiment. He is one member of the group of scientists trying to detect WIMPs as they traverse large, underground detectors. Other scientists hope to create them in powerful particle collisions at CERN’s Large Hadron Collider. “Most supersymmetric theories estimate the mass of the lightest WIMP to be somewhere above 100 gigaelectronvolts, which is well within LHC’s energy regime,” Tait says. “I myself and others are very excited about the recent LHC restart (http://www.symmetrymagaZine.org/article/june-2015/lhc- arrives-at-the-next-energy-frontier). There is a lot of hope to create dark matter in the lab.” (https://www.symmetrymagaZine.org/sites/default/les/images/standard/WIMPsLHC.jpg) A third way of searching for WIMPs is to look for revealing signals reaching Earth from space. Although individual WIMPs are stable, they decay into other particles when two of them collide and annihilate each other. This process should leave behind detectable amounts of radiation. Researchers therefore point their instruments at astronomical objects rich in dark matter such as dwarf satellite galaxies orbiting our Milky Way or the center of the Milky Way itself. (https://www.symmetrymagaZine.org/sites/default/les/images/standard/WIMPsSignals.jpg) “Dark matter interacts with regular matter through gravitation, impacting structure formation in the universe,” says Risa Wechsler, a researcher at Stanford and SLAC. “If dark matter is made of WIMPs, our predictions of the distribution of dark matter based on this assumption must also match our observations.” Wechsler and others calculate, for example, how many dwarf galaxies our Milky Way should have and participate in research eorts under way to determine if everything predicted can also be found experimentally. So how would researchers know for sure that dark matter is made of WIMPs? “We would need to see conclusive evidence for WIMPs in more than one experiment, ideally using all three ways of detection,” Wechsler says. In the light of today’s mature detection methods, dark matter hunters should be able to [nd WIMPs in the next [ve to 10 years, Shutt, Tait and Wechsler say. Time will tell if scientists have the right idea about the nature of dark matter. popular on symmetry (/article/april-2015/ten-things-you- (/article/ve-facts-about-the-big- might-not-know-about-antimatter) bang) 04/28/15 08/23/16 Ten things you might not know about Five facts about the Big Bang (/article antimatter (/article/april-2015/ten-things- /Fve-facts-about-the-big-bang) you-might-not-know-about-antimatter) It’s the cornerstone of cosmology, but what is it Antimatter has fueled many a supernatural tale. all about? It's also fascinating all by itself. (/article/dark-matter-day-recap) (/article/something-borrowed) 11/06/17 11/07/17 An international celebration of dark matter Something borrowed (/article/something- (/article/dark-matter-day-recap) borrowed) Around the world, scientists and non-scientists SLAC engineer Knut Skarpaas designs some of alike celebrated the [rst international Dark physics’ most challenging machines, [nding Matter Day. inspiration in unexpected places. Copyright 2016 Symmetry MagaZine A joint Fermilab/SLAC publication Terms of Use (/node/48371) (/) Ann. RelJ. Astron. Astrophys. 1987.25: 425-72 Copyright © 1987 by Annual Reviews Inc. All rights reserved EXISTENCE AND NATURE OF DARK MATTER IN THE UNIVERSE Virginia Trimble Astronomy Program, University of Maryland, College Park, Maryland 20742, and Department of Physics, University of California, Irvine, California 92717 1. HISTORICAL INTRODUCTION AND THE SCOPE OF THE PROBLEM The first detection of nonluminous matter from its gravitational effects occurred in 1844, when Friedrich Wilhelm Bessel announced that several decades of positional measurements of Sirius and Procyon implied that each was in orbit with an invisible companion of mass comparable to its own. The companions ceased to be invisible in 1862, when Alvan G. Clark turned his newly-ground 181" objective toward Sirius and resolved the 10-4 of the photons from the system emitted by the white dwarf Sirius B. Studies of astrometric and single-line spectroscopic binaries are the modern descendants of Bessel's work. A couple of generations later, data implying nonluminous matter on two very differentscales surfaced almost simultaneously. First, Oort (498, 499) analyzed numbers and velocities of stars near the Sun and concluded that visible stars fell shy by 30-50% of adding up to the amount of gravitating matter implied by the velocities. Then, in 1933, Zwicky (777) concluded that the velocity dispersions in rich clusters of galaxies required 10 to 100 times more mass to keep them bound than could be accounted for by the luminous galaxies themselves. The former result was taken much more seriously than the latter by contemporary and succeeding astronomers (being dignified by the name "the Oort limit"), which is perhaps more a statement about the personalities ofOort and Zwicky than about anything else. 425 0066--4146/87/091 5--0425$02.00 426 TRIMBLE The next decades were by no means devoid of relevant ideas and inves­ tigations (346, 484). The beginning of the modern era of dark-matter research can, however, be dated to 1974, when Ostriker, Yahil & Peebles (506) and Einasto, Kraasik & Saar (193) tabulated galaxy masses as a function of the radius to which they applied and found M increasing 12 linearly with R out to at least 100 kpc and 10 M 0 for normal spirals and ellipticals. Since then, a mainstream astronomer who seriously doubted that we are somehow not readily seeing 90% or more of the stuffin the Universe has found himself in the position of having to justify his discordant views. The low-mass torch, upheld for a time by Burbidge (109) and Woltjer (755), has recently been refueled by Valtonen (706-708). Because dark matter has been invoked in many differentobjects and on many differentscales, a very large fraction of astronomical research bears in some way on the issue. Necessarily, then, many aspects are given rather short shrift here. First, nothing is said about the value of the Hubble constant, though it enters in powers from -2 to + 2 into various determinations of mass and luminosity of distant objects and is arguably the largest single uncertainty in these determinations (268). Hodge's (301) 1981 conclusion that an impartial choice of value for H 0 would be both difficult andunprofitable remains regrettably correct. Besides, like Hodge, I have friends in both camps. Next, several other relevant topics that have recently been reviewed in this series are somewhat neglected.
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